PROJECT SUMMARY Hijacking of normal developmental programs is a common mechanism of tumorigenesis and deregulation of developmental HOX programs, in particular, contributes to the pathogenesis of leukemia, as well as some solid tumors. We recently discovered that HOX gene expression is highly deregulated in Ewing sarcoma, an aggressive bone and soft tissue tumor that has a peak incidence in adolescence. Specifically, the posterior HOX genes, HOXD10, HOXD11 and HOXD13, are over-expressed compared to other normal and malignant tissues, and down-regulation of HOXD13 in Ewing sarcoma cell lines results in a dramatic decrease in tumorigenicity. The expression of all Hox genes normally becomes restricted to distinct regions of the developing musculoskeletal (MSK) system during embryonic development, with the posterior genes becoming restricted to the lumbosacral vertebrae, pelvis and developing forelimb and hindlimb ? the dominant sites of Ewing sarcoma presentation clinically. Recent work from the Wellik laboratory has shown that Hox-expressing cells arise as mesenchymal progenitor/stem cells (MSCs) during embryonic development and persist as regionally-restricted MSCs through postnatal and adult stages. MSCs have been implicated as putative cells of origin for Ewing sarcoma, but the precise identity of the target cells, the molecular mechanisms of malignant transformation and a clear explanation of the region-specific etiology of this tumor type remain poorly understood. Important for this application, the Lawlor lab has shown that ectopic expression of the Ewing sarcoma driver oncogene, EWS/FLI1, in bulk populations of MSCs initiates tumorigenesis and disrupts HOX gene expression as a result of epigenetic deregulation. These two key findings lead us to the novel hypothesis that EWS/FLI1-dependent disruption of the posterior HOX program in MSCs where these genes are developmentally and regionally expressed (i.e. the posterior skeleton) is central to Ewing sarcoma tumorigenesis. We will determine the efficiency and latency of EWS/FLI1-induced tumor formation in Hox-expressing MSCs that are isolated from different anatomic sites and test the hypothesis that oncogenic transformation occurs more readily in MSC populations where restricted, posterior Hox gene expression has been established. We will use Hoxa11eGFP reporter mice to monitor ectopic induction of the posterior Hox program in transduced cells and their resultant tumors. Use of MSCs from control and Hox mutant mouse lines will allow us to determine if posterior Hox genes are functionally required for the transformation process in vivo as our in vitro data suggest. With the biochemical, molecular and in vivo genetic tools the two labs have developed, successful demonstration of this novel hypothesis during this funding period will provide the basis for future highly impactful and mechanistic dissection of the etiology of Ewing's sarcoma, including a molecular basis for understanding the regional specificity of this tumor in vivo.